1. Field of the Invention
This invention relates generally to multipurpose switching circuits used, for example, to from a synchronous detector, and more particularly to solid state multipurpose switching circuits employing a novel semiconductor device of the three-terminal type with a superior symmetrical characteristic.
2. Description of the Prior Art
Several types of multipurpose switching circuits have been proposed for use as, for example, a synchronous detector such as a color demodulator for demodulatng a chrominance signal of a composite color television signal which is amplitude-modulated on a modulation axis of a predetermined phase. In such circuits, at least one switching element is provided and a first signal which is to be switched is applied to an input terminal connected to the switching element. The switching element is supplied with a second signal and controls the switching operation in accordance with the second signal so as to transmit the first signal to an output terminal intermittently during its conductive or nonconductive state, thereby to produce the switched first signal which is switched in synchromism with the second signal.
There has further been a multipurpose switching circuit of a different type which has a couple of input terminals and at least one single output terminal connected to a switching element to derive at the output terminal a composite of the first and second signals respectively applied to both input terminals alternatively in accordance with a third signal. The third signal is supplied to the switching element to control the switching operation.
Such multipurpose switching circuits as described above are usually constructed in the form of a balanced type, that is, a pair of signal transmitting paths including a switching element are symmetrically provided with a common output terminal and alternately switched by a switching signal, so as to avoid a change in the D.C. level of the output signal which tends to occur in response to the switching operation of the switching element. Accordingly the circuits usually require a number of elements performing the switching or ON-OFF operation and moreover are relatively complicated in structure.